It's not emotional it's common sense. Nothing proven means you don't know what you are shooting at, period.= unethical. Couple that with the fact it is standing and or running/walking on two feet and probability dictates it is most likely a human being. Smeja relates a great deal of internal conflict over this. He ultimately rejected the thought it was human due to circumstances and took the shot, if we are to believe the story. Whether you agree with the decision or not, he didn't know what he was shooting, his best guess was that it wasn't human, until apparently the little one died in his arms.
What about the guy in a Gillie suit? how many times could you use that plea?